1. Field of the Invention
This invention relates to a receptacle for the reception, vaporization and atomization of analysis samples for atomic absorption spectroscopy.
2. Description of the Prior Art
In flameless atomic absorption spectroscopy, the analysis sample introduced for example through a lateral opening into a tubularly extended receptacle is evaporated and yields an "atom cloud" consisting essentially of free atoms which is irradiated in the direction of the tube axis with a measured light beam containing resonance lines of the element which is sought. The temperatures required for drying, ashing, vaporization and atomization are produced by Joule's law heating of the receptacle which consists of a temperature resistant conductor, preferably made of graphite or other carbon types of material, such as pyrographite, vitreous carbon, and as a rule is connected via contact pieces with an electrical supply arrangement. The factors determining precision of analysis are, inter alia, the residence time of the vaporized analysis substance in the receptacle, the atomization time and the ratio of the two measurements. Numerous proposals have been made for shortening the time span necessary for the atomization, above all by higher rate of heating. For example, it is known, to reduce the losses caused by radiation, which losses limit the heating rate, by encasing the receptacle with jacket-like insulating bodies. Another proposal was to restrict the current flow through a layered construction to a narrow zone extending from the inner wall (German Offenlegungsschrift No. 31 40 458). Another essential requirement for the precision of analysis is a small temperature fluctuation over the volume of the receptacle. Especially harmful are, in this connection, matrix effects which achieve reversal to reactions of the desired elements with other components of the analysis sample and formation of compounds which are resistant in zones of lower temperature. At the least, one can reduce, in part, the harmful effects by additions of specific reagents or calibrating substances and even by selective evaporation. The cost of this, for its part, is large, without the reliability of analysis being essentially increased.
Finally, it is known from U.S. patent specification No. 4,407,582 to heat the tubular ends of receptacles alone by direct current passage alone and to heat the center thereof, proceeding from the ends of the receptacle by means of heat conduction and radiation. The energy supplied naturally must suffice for the decomposition and atomization of the analysis sample. No temperature sinks are to exist in the direction of the ends of the receptacle. Electrical energy is supplied to the ends of the receptacle over Y-shaped contact pieces or a slotted casing which connects to special shoulders of the receptacle ends.
The matrix effect is clearly less with these constructional forms than with tubular receptacles with central current supply. A disadvantage of the arrangement is the restriction of the rate of heating as a result of the permitted current densities of the contact pieces. The receptacle ends can become exposed, without being destroyed, by evaporation of the carbon, peeling off of graphite flakes or the formation of cracks. The supply of sufficient energy to the core zone by Joulean heat and irradiation for the atomization of all test substances and the suppression of the matrix effect are therefore not always successful. Even with normal electrical current density, the contacts are strongly stressed and a rapid wearing may be difficult to avoid.
The disadvantages of this solution are to a large extent avoided with an arrangement in which different heating circuits are provided for the tubular graphite receptacles and for a special cup for the reception of the substance to be analyzed. The receptacle is initially heated up to the predetermined temperature and after reaching this temperature the cup inserted in an opening in the receptacle is heated with the substance to be analyzed with a high rate of heating to the atomization temperature (Spectrochimica Acta 37B, 1021, 1982). Within one to two seconds, temperatures of about 2700.degree. C. are achieved and a constancy of temperature with respect to both space and time, improving the precision of analysis, is achieved. Even with this constructional arrangement, the alteration with time of the contact positions between receptacles or cups and the graphite rods serving for the supply of current is technically not satisfactory. It is especially difficult reproducibly to attach the contact rods. Different contact resistances exist too as a result of variations in the shape of the contact partners as a consequence of steep variations in temperature. The described effects reduce the operating life of the system which includes the receptacle and make routine analyses difficult.